I don't think he meant it in a strict mathematical sense. Just that it's easy/natural to pick a direction on a line and say point C is after A, and B is in-between; whereas on a surface there isn't an obvious ordering.
It however isn't difficult to do this for a 3-axis machine. in fact, the way he describes building layer by layer is exactly how you make complex shapes on a 3-axis mill. You essentially trace a line on the surface that you are trying to machine. The problem simply reduces to the same line problem with the points defined in 3 dimensions.
Of course, there are notable complexity benefits of 3D printing over 3-axis machining, but many of them can be overcome with other machining methods such as EDM.
